Fix deadlock after parsing directory with no files

Currently if interpreter tries to eval an empty directory with no Go files it will error (as it should), but all future calls to eval will deadlock, because the mutex is not unlocked correctly. I believe this is a critical issue that must be addressed.

interp/src.go

@@ -133,6 +133,7 @@ func (interp *Interpreter) importSrc(rPath, importPath string, skipTest bool) (s
 	interp.mutex.Lock()
 	gs := interp.scopes[importPath]
 	if gs == nil {
+		interp.mutex.Unlock()
 		// A nil scope means that no even an empty package is created from source.
 		return "", fmt.Errorf("no Go files in %s", dir)
 	}